Subscribe to Spaceflight Now Plus for access to our extensive video collections!How do I sign up?Video archiveDay 10 highlightsVideo highlights from Flight Day 10 as the astronauts enjoyed a mid-mission light-duty day.PlaySTS-123 day 9 highlightsFlight Day 9 saw Dextre moved to the Destiny lab, stowing the assembly pallet and a group dinner in the Zvezda module.PlaySTS-123 day 8 highlightsFinishing assembly of Dextre and delivering spare parts to the station occurred during the spacewalk on Flight Day 8.PlaySTS-123 day 7 highlightsThe space station's handyman robot Dextre flexed its arms for the first time during Flight Day 7.PlaySTS-123 day 6 highlightsFlight Day 6 was dedicated to the second spacewalk and continued assembly of the Dextre robot.PlaySTS-123 day 5 highlightsThe station and shuttle crews opened up and entered Japan's new Kibo logistics module during Flight Day 5.PlaySTS-123 day 4 highlightsHighlights of the Kibo logistics module's attachment to the station and the first spacewalk to begin Dextre assembly.PlaySTS-123 day 3 highlightsThis movie shows the highlights from Flight Day 3 as Endeavour docked to the space station.
PlaySTS-123 day 2 highlightsFlight Day 2 of Endeavour's mission focused on heat shield inspections. This movie shows the day's highlights.
PlaySTS-123 day 1 highlightsThe highlights from shuttle Endeavour's launch day are packaged into this movie.
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With the shuttle Endeavour's mission entering the home stretch, shuttle Discovery remains on track for blastoff May 25 to ferry a huge Japanese laboratory module to the international space station. But subsequent near-term flights, including a high-profile mission to service the Hubble Space Telescope, could be delayed, sources say, because of ongoing external tank production issues.

The tank used by Endeavour for its current mission was the last in the inventory of tanks built before the 2003 Columbia disaster and subsequently modified to reduce potentially dangerous losses of foam insulation. The tank slated for use with Discovery in late May, ET-128, is the first so-called "in-line" external tank built from the ground up with post-Columbia upgrades, including a new ice-frost ramp design and titanium oxygen line support brackets. Both improvements address areas of possible foam shedding.

ET-128 for Discovery's upcoming mission leaves the Test and Check-out Building near New Orleans this week for the journey to Kennedy Space Center. Credit: Lockheed Martin

ET-128 departed Lockheed Martin's Michoud Assembly Facility in New Orleans Thursday for the 900-mile barge trip to the Kennedy Space Center in Florida.

But a backlog of work at Michoud is hampering downstream tank deliveries. The tank that would be needed for a rescue mission should some mishap strand Discovery's crew in orbit is not expected to reach the Kennedy Space Center until late summer. NASA managers say the space station has enough supplies on board to support a combined crew for more than three months if necessary and as of this writing, Discovery's launch remains on track.

But the picture is cloudier for NASA's next shuttle flight, a mission by the shuttle Atlantis to service the Hubble Space Telescope. Launch currently is targeted for Aug. 28. ET-127, the tank designated as the emergency backup for Discovery's May mission, is the prime tank for the Hubble flight.

Safe haven aboard the space station is not an option for Atlantis' crew if major heat shield damage occurs. The observatory and the station are in different orbits and the shuttle does not have the ability to move from one to the other. As a result, NASA Administrator Mike Griffin early on approved plans to have a second shuttle, Endeavour, ready for launch on a rescue mission just in case.

That means NASA needs two ready-to-fly external tanks for the Hubble mission, ET-127 and ET-129 respectively. Manpower and production issues, triggered in part by unplanned work to upgrade low-level hydrogen fuel sensors and other post-Columbia design upgrades, have slowed external tank manufacturing and sources say the Hubble mission faces a possible delay to October.

Senior program managers visited Michoud for a first-hand look earlier this week and a more realistic assessment of the tank production schedule is expected in the next week or so. For now, the Hubble mission remains officially targeted for launch Aug. 28.

Shuttle Program Manager John Shannon told CBS News Thursday that NASA has "added many new features to further ensure the safety of this tank and since these were the first in-line tanks, we took extra time to make sure we got it right."

"We have margin in the schedule to absorb small delays and we have learned a lot in the process of putting these tanks together so that the '09 and '10 tanks will flow much faster," he said.

Shannon did not address specific launch dates.

NASA plans to complete the space station and retire the shuttle by the end of fiscal 2010. The current manifest calls for four more flights this year - in May, August, October and December - four in 2009 and up to three in 2010.